A multi-wavelength, high bandwidth (1 Ghz) frequency-domain photon migration (FDPM) instrument has been developed for quantitative, non-invasive measurements of tissue optical and physiological properties. The instrument produces 300 kHz to 1 Ghz photon density waves (PDWs) in optically turbid media using a network analyzer, an avalanche photodiode detector, and four amplitude-modulated diode lasers (674 nm, 811 nm, 849 nm, and 956 nm). The frequency-dependence of PDW phase and amplitude is absorption, ma, and reduced scattering, ms', parameters. The wavelength-dependence of absorption is used to determine tissue hemoglobin concentration (total, oxy- and deoxy-forms), oxygen saturation and water concentration. We present preliminary results of non-invasive FDPM measurements obtained from normal and tumor-containing human breast tissue. Our data clearly demonstrate that physiological changes caused by the presence of small (about 1-2 cm diameter) palpable lesions ca n be detected using a hand-held FDPM probe.